Analog wireless communications systems have been long established in the United States and elsewhere. For example, the advanced mobile phone service (AMPS) cellular telephone system was developed in the early 1970s and is still in widespread use in the United States today. The AMPS system uses Frequency Division Multiple Access (FDMA) to provide shared access to the available communications spectrum. With FDMA, users are assigned to a channel from a limited set of communications channels in the available communications spectrum. In typical AMPS systems, 25 kHz communications channels are used whose carriers are separated by 30 kHz. The number of available channels is limited by the size of the available communications spectrum. When a user places a call, the user is assigned to one of the available channels. If no channel is available, the user's call is blocked. Unfortunately, the increasing demand for cellular services has resulted in a corresponding increase in the frequency of call blocking, particularly in congested urban areas.
Recent advances in digital communications allow for an increase in network. capacity without requiring allocation of additional communications spectrum. In addition to FDMA, digital wireless communication systems use time division multiple access (TDMA) or code division multiple access (CDMA) to allow multiple users to share the same carrier channel frequency, thereby increasing network capacity. In a TDMA system, for example, a single RF carrier is divided into time frames of a pre-defined length. Each time frame is further divided into plural time slots, each time slot representing a separate communications channel. A plurality of users can transmit and receive data in short bursts in respective time slots, thereby allowing a plurality of users to share a single carrier channel frequency.
Another advantage of digital systems is improved voice quality. In an analog system, physical influences or disturbances in radio transmission links are sometimes passed into the audio path of the receiver, creating static, hums, hisses, crackling sounds, cross-talk and fade-outs in the received voice signal. In digital communication systems, the audio signal is transformed into digital data patterns and digital coding and error correction methods allow the audio signal to be reconstructed at the receiving end of the transmission. Digital signal processing techniques eliminate many of the effects of disturbances in the radio transmission link.
Because of these and other advantages of digital communications, there has been a move to retire existing analog cellular systems in favor of digital cellular systems. However, there is a large investment in the current analog infrastructure and it is not practical to completely abandon existing analog systems. Moreover, there are millions of existing analog-only mobile phones in use which require analog infrastructure to operate. The transition to digital systems will likely take many years or decades. Consequently, it can be expected that cellular services in the U.S. will consist of a mixture of analog and digital systems.
Several dual-mode systems have been developed which handle both analog and digital communications. In the United States, the predominant dual-mode systems are operated according to the protocol described in Telecommunications Industry Association Interim Standard IS-136. Within IS-136 and similar systems, digital communications are more desirable, but both analog and digital modes are supported.
Analog, digital, and dual-mode wireless communications systems typically employ a large number of discrete communication channels for communicating voice and data from one location to another and for controlling system functions. Further, it is also common for there to be multiple communications service providers in competition with each other for each type of communication service. As such, within a given geographical area it is typical for the various communication channels to be allocated to the different competitors such that only one service provider is responsible for providing service on any given channel.
To facilitate communications channel allocation, the spectrum of channels may be divided into several channel bands, each containing a plurality of communications channels. Thereafter, an entire channel band may be allocated to a particular service provider for a given geographic region. Thus, in a certain region, service provider Alpha may be allocated channel bands A and C, while service provider Beta may be allocated channel band B, and service provider Gamma all other channel bands. The particular details of the channel band allocations are well known in the industry.
The various service providers are typically in competition with each other. However, many service providers have bi-lateral agreements under which subscribers (users) to one service provider may use the communications network of another service provider. Despite the prevalence of such agreements, some service providers have no such agreements with particular other service providers for competitive reasons. For example service provider Alpha may have a sharing agreement with service provider Beta, but no agreement with service provider Gamma. Users who are subscribed to Alpha may use communications channels belonging to service provider Beta, but should not use any channels belonging to service provider Gamma. In order to ensure this, the Alpha mobile units are typically given a list of service providers that are forbidden to be used; in this example, the list would include Gamma but not Beta.
During use, it is common for the mobile unit to be directed from one channel to another, an event which can be triggered by a wide variety of reasons. For example, a dual-mode cellular phone may initially camp on an analog control channel. Because digital mode operation is preferred, it is common for a digital control channel pointer to be broadcast periodically on the analog control channel when the wireless communications system is dual-mode capable. When the phone detects the digital control channel pointer, the mobile phone ideally camps on the pointed-to digital control channel and operates thereafter in digital mode.
Unfortunately, in some instances, the digital control channel pointer may be pointing to a digital control channel that belongs to a service provider that the phone should not be using. In such situations, the prior art suggests that the mobile phone should initiate a power-up scan rather than camp on the undesirable digital control channel. However, if the mobile phone is camped on an analog control channel, it is probable that the mobile phone found no suitable digital control channel during the last power-up scan. As such, it is likely that the mobile phone will simply end up camping on the same analog control channel after the power-up scan. If so, the power-up scan has served no useful purpose. Because a power-up scan utilizes substantial system resources including mobile unit power, it is desirable to avoid useless power-up scans.
Therefore, it is desirable for mobile phone to avoid power-up scans when a digital control channel pointer is received on an analog control channel that points to a control channel belonging to a service provider that the mobile phone is forbidden to use.